1. Field of the Invention
The invention relates to a current limiting switch capable of rapid circuit breaking, and more particularly to such a switch including at least one pair of mutually cooperating contacts, one contact of the pair being mounted fixedly and the other one thereof being mounted on a contact arm, the contact arm being operable to rotate about a fixed point and upon surpassing a predetermined current threshold value being destined to be disengaged from the fixed contact in a pivoting movement about a fixed point by electromagnetic means energized by the current passing through the switch. The electromagnetic means comprising an electromagnet and a first and a second armature cooperating with the electromagnet, in which the first armature is operable upon being energized to drive the movable contact in the direction of disengagement and the second armature is operable to release a locking device. The locking device being operable to block the movable contact in the direction of movement until the current threshold is reached. The locking device includes a locking arm operable to rotate about a second fixed point and to forestall the movement of the contact arm in the direction of disengagement in its resting position as long as the current threshold is not surpassed and in which the first and the second armature, respectively, cooperate with the contact arm and the locking arm. The free end of the locking arm in the resting position thereof facing a cam on the contact arm in the direction of rotation in which the contact arm is moved upon energization of the electromagnet.
2. Prior Art
A current limiting switch of this type is known from the French Patent Specification No. 1,246,837. This switch is provided with a single electromagnet operable to attract both a contact armature for breaking the circuit and a second armature for disengaging the locking means, the locking means releasing the contact armature upon disengagement of the locking means when the current through the switch surpasses the predetermined current threshold value. Until the moment of release of the contact a sufficient contact force is maintained so that a sharp definition of the threshold value becomes possible while a welding of the current carrying contact is prevented. This known switch presents, however, the drawback that the contact armature will be kept in the closed contact position until released by the locking means. Upon release the contact armature will fly to its current breaking position so that both in the non-released condition of the armature and upon reaching the final circuit breaking position of the armature the pivot determining the rotary movement of the armature will be subject to high inhomogeneous stresses. This may even be amplified in case of serious fault conditions in which magnetic contact blowoff forces or repulsion forces occur between the contacts.